When do you sell a stock?

When a stock you've bought for its perceived value underperforms, how long do you wait before selling? What's your rule of thumb for cutting losses and freeing up capital for potentially better investments? How do you identify a truly unrecoverable investment?

Warren Buffett seems to never do it, but honestly it seems practically impossible, isn't it?
https://blog.valuesense.io/understanding-buffett-when-to-sell-stocks-hint-its-never/